1. Field of the Invention
The present invention relates to an acceleration sensor used in a moving object such as vehicles and the like, and more particularly to an acceleration sensor capable of detecting the linear acceleration of the moving object based on variations in capacitance of a capacitor mounted on the moving body, and also to a multi-axis acceleration sensor capable of detecting the linear acceleration of the moving object in a plurality of directions and the angular velocity of the moving object, and further relates to the method of producing the acceleration sensors.
2. Description of the Prior Art
In recent years, in order to improve automobiles in safety and in amenity in their driving operation, there are provided various types of control systems such as active suspension systems, air-bag systems, anti-skid brake systems and the like. For these systems, the linear acceleration and the angular velocity of the automobiles are indispensable information, and, therefore detected by the use of acceleration sensors mounted on the automobiles.
Of these acceleration sensors, for example, one disclosed in Japanese Patent Laid-Open No. Hei 4-252961 is of a capacitor type constructed of a weight and a board member oppositely disposed from the weight. In operation, this one calculates variations in capacitance of the thus constructed capacitor to determine both the angular acceleration and the linear acceleration of the automobile. Disclosed in this Japanese Patent Laid-Open No. Hei 4-252961 are: an acceleration sensor permitting a weight to be displaced about the X axis or the Y axis; and, an acceleration sensor permitting the weight to be displaced about both the X and the Y axis.
Of these acceleration sensors, the former one is constructed of a flat rectangular weight and a frame-like base board (i.e., board member), the rectangular weight having a center or intermediate point of each of its opposite long sides supported by the frame-like base board through a supporting beam. In the acceleration sensor: the weight is slightly thinner than the base board; the supporting beam axially symmetrically supports the weight so as to permit the weight to be rotated to a limited extent on its center; in each of a front and a rear surface of the weight, there are provided a pair of fixed electrodes, the fixed electrodes being spaced apart from each other at predetermined intervals; and, disposed in each of a front and a rear surface of the frame-like base board is a plate of glass (i.e., glass plate), provided inside which glass plate are additional fixed electrodes corresponding in position to the above fixed electrodes, so that four capacitors in total are constructed of these fixed electrodes thus oppositely disposed from each other. In operation, when the weight is displaced about the X axis or the Y axis, the capacitors vary in capacitance. Such variations in capacitance of the capacitors are calculated to determine the angular acceleration and the linear acceleration of the moving object.
On the other hand, in the latter one of the acceleration sensors: a first through-hole is formed in a central portion of the frame-like base plate assuming a rectangular shape; a first frame-like weight, which is thinner than the base board, is disposed in the base board so as to be supported by the base board through a first supporting beam, which beam extends in a direction parallel to the Y axis passing through the center of gravity of the weight; a second flat weight, which assumes a rectangular shape extending in a direction parallel to the Y axis, is disposed in a second central through-hole of the first weight; and, the second weight is supported by the first weight through a second supporting beam, which beam extends in a direction parallel to the X axis passing through the center of gravity of the second weight. The first weight is capable of rotating on the Y axis (i.e., on the first supporting beam) to a limited extent relative to the base board, and has its long portion serve as a weight which is longer than the second through-hole in a direction parallel to the X axis. The second weight is capable of rotating on the X axis (i.e., on the second supporting beam) to a limited extent relative to the first weight. In each of the front and the rear surface of each of the first and the second weight, there are provided a pair of fixed electrodes, the fixed electrodes being spaced apart from each other at predetermined intervals. In each of the inner surfaces of the glass plates disposed in each of the front and rear side of the frame-like base plate, there are provided a plurality of additional fixed electrodes which are oppositely disposed from the above corresponding fixed electrodes to form six capacitors in total therebetween. When the first weight is displaced about the Y axis and the second weight is displaced about the X axis to produce a resultant displacement, each of the above capacitors vary in capacitance according to such resultant displacement. Consequently, it is possible to determine the angular acceleration and the linear acceleration of the moving body by calculating thus obtained variations in capacitance of these capacitors.
In the former one of the conventional acceleration sensors, the supporting beam is provided in the side surface of the weight so as to extend outward (i.e., toward the base board). Consequently, it is necessary for the former one to provide some space outside the side surface of the weight so as to receive the supporting beam therein, which makes it difficult to reduce the former one in size. When the supporting beam is reduced in length in order to realize the down-sizing of the former one, the thus shortened supporting beam is poor in fatigue strength, and, therefore is susceptible to cracks and fractures when the weight is displaced to stress the supporting beam.
On the other hand, in the latter one of the conventional acceleration sensors, there are provided the electrodes, which ones are formed in each of the first and the second weight separately. Consequently, in case that the electrodes are formed by the use of graphic arts, it is necessary for the electrodes to be formed in each of the first and the second weight separately, which complicates the manufacturing process of the electrodes. Further, since each of the electrodes requires its own wiring, the arrangement of such wiring is also complicated.
Still further, in the latter one of the conventional acceleration sensors: the first and the second weight are disposed in an inner and an outer position, respectively; these weights are different in size, and provided with the electrodes separately to form the capacitors therebetween: the first and the second weight are different in moment energy due to the difference in size therebetween; whereby, even when the first and the second weight are displaced with the same angular acceleration about the X axis and the Y axis, some errors are produced in variations of the capacities of the capacitors constructed of the electrodes. Consequently, as for the latter one of the conventional acceleration sensors, in order to realize a precision acceleration sensor, it is necessary in practice to additionally provide some measures for compensating for such errors in angular acceleration due to the difference in moment energy between the first and the second weight, the measures comprising: a proper redesigning of the supporting beam (i.e., by adjusting the supporting beam in diameter and like parameters to compensate for the errors); and, a suitable measurement circuit for compensating for the errors. However, such additional provision of the measures increases the manufacturing cost.
On the other hand, for example, in Japanese Patent Laid-Open No. Hei 4-252961, there is disclosed an acceleration sensor in which: a board-like weight made of electrically-conductive material is provided with an axle and mounted in the sensor so as to be rotatable on the axle; a fixed electrode is provided so as to be oppositely disposed from the weight, so that a capacitor is constructed of the weight and the fixed electrode; whereby, when the sensor is subjected to a linear acceleration, the weight rotates on its axle to cause the capacitor to vary in capacitance, the variation in capacitance of the capacitor being measured to determine the linear acceleration.
Another conventional acceleration sensor is disclosed in Japanese Patent Laid-Open No. Hei 5-256841, in which: a movable electrode is supported by a cantilever, and disposed between a pair of fixed electrodes to form a pair of capacitors therebetween. When the sensor is subjected to a linear acceleration, the movable electrode is displaced so that the capacitors vary in capacitance, whereby the linear acceleration is determined based on such variations in capacitance of the capacitors.
Three-dimensional movement of the moving object is represented by coordinates (i.e., of x, y and z positions) of the moving object in the three-dimensional space and angular velocities in rotational movement (i.e., pitching, yawing, rolling) of the moving object. In the above-mentioned various control systems of the automobiles, it is desirable to use all the above-mentioned parameters.
For example, in the above-mentioned active suspension system and the anti-skid brake system, both the linear acceleration and the angular acceleration are required.
Consequently, as for the acceleration sensor mounted on the automobile, it is desirable to use a single acceleration sensor which is capable of detecting the angular acceleration and the linear acceleration in each of a plurality of directions.
However, any of the above-mentioned conventional acceleration sensors is capable of detecting the linear acceleration in only one direction together with the angular acceleration. Consequently, when it is necessary to determine the angular acceleration and the linear acceleration of the moving object in three directions (i.e., in directions parallel to the X, Y and the Z axis) in the three-dimensional space, both a linear acceleration sensor and an angular acceleration sensor are required as to each of the directions, which makes the conventional acceleration sensor very disadvantageous from the economical point of view. Further, this increases the number of components of the conventional acceleration sensor, which makes the sensor poor in reliability.